1. Field of the Invention
This invention relates to an optical communication device (LD module, PD module, LD/PD module) for transmitting and receiving multichannel signals by M-channel ribbonfibers (tapefibers) which contain M element fibers, where M is the number of channels. The pitch of multichannel fibers has been determined to be 250 xcexcm. Optoelectronic device chips, laser diodes or photodiodes, are squares of a side from 600 xcexcm to 300 xcexcm.
Sizes of optoelectronic devices are larger than the pitch of the multichannel fibers. Size discrepancy inhibits optical communication modules from installing M optoelectronic devices on straight line extensions of the M element fibers of the M-channel ribbonfibers.
This application claims the priority of Japanese Patent Application No.2002-153870 filed on May 28, 2002, which is incorporated herein by reference.
2. Description of Related Art
FIG. 5 shows an MT connector of a standard type of a four channel connector. A resin-made MT male connector 52 has a pair of guidepins 54 and 54 projecting from the front. The MT connector 52 contains a front end of a four-core ribbon fiber 56. The four-channel ribbonfiber 56 includes four optical fibers FBa, FBb, FBc and FBd. The standardized fiber pitch P1 of the ribbonfiber is 250 xcexcm (0.25 mm). Four ends of the fibers appear on the front surface.
Another resin-made MT female connector 53 has a pair of guideholes 55 and 55 for joining to the above male connector 52. The MT connector 53 contains a four channel ribbonfiber 56 including four element fibers. Both connectors are combined or separated by the guidepins and holes. When the connectors 52 and 53 are coupled with each other by inserting the guidepins into the guideholes, each end of the element fibers of the connector faces an end of a counterpart fiber of another connector. The MT connector is one of low-cost, small-sized, prevalent connectors. The fiber pitch P1 is 250 xcexcm. Laser diode chips or photodiode chips are squares or rectangles of a side of 600 xcexcm to 300 xcexcm. It is impossible for a multichannel fiber to join laser diodes or photodiodes to the element fibers with keeping the pitch P1.
{circle around (1)} M. Shishikura, K. Nagatsuma, T. Ido, M. Tokuda, K. Nakahara, E. Nomoto, K. Sudoh and H. Sano, xe2x80x9c10 Gbpsxc3x974-channel parallel LD modulexe2x80x9d, Proceeding of the 2001 IEICE, C-3-50, p160
FIG. 6 shows a perspective view of a 10 Gbps four channel LD module with enlarging lightwaveguides proposed by {circle around (1)}. The LD module is made on a silicon bench 57 by piling a SiO2 lightwaveguide layer on the silicon bench, producing four width enlarging lightwaveguides Wa, Wb, Wc and Wd having a 250 xcexcm initial pitch and a 1000 xcexcm final pitch and installing four laser diodes LDa, LDb LDc and LDd at final ends of the lightwaveguides at a 1000 xcexcm pitch.
The initial pitch P1 of the lightwaveguides Wa, Wb, Wc and Wd is 250 xcexcm for coinciding with the pitch of multichannel ribbon fibers which contain M element fibers at a 250 xcexcm pitch. The initial 250 xcexcm pitch allows the ribbonfiber to adhere to a front end of the lightwaveguides permanently. The final 1000 xcexcm pitch allows individual laser diodes LDa, LDb, LDc and LDd to align at ends of lightpaths with sufficient margins. Smooth curvatures enlarge a width of the lightpaths continuously from the 250 xcexcm pitch to the 1000 xcexcm pitch on the silicon bench.
Parallel arrangement of a plurality of laser diode chips of a square from 600 xcexcm to 300 xcexcm requires a wide pitch more than 500 xcexcm for the sake of thermal diffusion and a chip installment space. A wide pitch more than 1000 xcexcm is preferable for depressing mutual crosstalk. The known reference {circle around (1)} enlarges the pitch of lightwaveguides continuously in the lateral direction along the lightpaths for solving the problem of the pitch discrepancy and the crosstalk.
The pitch P1 of multichannel fibers is different from a pitch P2 for aligning a plurality of lasers or photodiodes on a bench of a surface-mounting type module. A channel pitch should be enlarged midway on lightpaths from a ribbonfiber to a multichannel optical communication module. It is a good contrivance of enlarging the channel pitch by making curving lightwaveguides on a silicon bench like the known reference {circle around (1)}. However, it is difficult to produce the curving lightwaveguides on the silicon bench with high preciseness. Moderate curvature of a lightwaveguide requires a large length of the silicon bench. Alignment of lasers or photodiodes with lightwaveguides is a difficult step, which raises the cost of modules. A long silicon bench and built-in curving lightwaveguides enhance the cost since P1 and P2 have a difference.
One purpose of the present invention is to provide an optical communication device which enlarges the multichannel pitch without using of the high-cost curving lightwaveguides. Another purpose of the present invention is to provide a device which alleviates the length of the silicon bench. A further purpose is to provide a device which dispenses with a time-consuming alignment step for joining fibers to bench-built in lightwaveguides. A further purpose is to provide a device which can easily be attached to or detached from a prevalent multichannel MT connector.
The present invention proposes a communication device containing a connector having a pair of guidepins or guideholes, maintaining ends of a channel number of parallel optical fibers having tails extending from a rear wall, a bench (e.g., silicon bench) having M (M=channel number) parallel V-grooves aligning at a pitch P2 wider than a pitch P1 at a forward part, M linear lightwaveguides aligning at P2 and following the V-grooves and laser diodes or photodiodes mounted at ends of the lightwaveguides and a package including leadpins for supporting the connector, the fibers and the bench. The width of neighboring fibers is enlarged from P1 to P2 at an enlarging space ES between the connector and the silicon bench.
The discrepancy of the pitches P1 and P2 requires a pitch conversion element between the connector and the lasers or photodiodes. The present invention allows the optical fibers to bend freely at the enlarging space ES for converging the pitch instead of built-in lightwaveguides. Lightpaths align at P1 in the connector but align at P2 on the bench. The present invention enlarges the pitch by freely bending the fibers at the free space ES from the P1 pitch to the P2 pitch. The optical fibers curve with a locus consisting of two reciprocal arcs. The joint of two arcs is a neutral point without curvature. An elastic fiber yields optimum curvature in a freestanding state. Formation of curvatures in a free space enjoys high degree of freedom unlike formation of lightwaveguides on a silicon bench. It is easy to bend thin, elastic fibers in the free space.
The pitch-conversion portion (ES) exists out of the silicon bench in the device of the present invention. The bench does not contain the pitch conversion portion. The silicon bench should have parallel straight lightwaveguides and parallel straight V-grooves in the longitudinal direction. The silicon bench is immune from curved lines. A plurality of silicon benches can be made upon a silicon wafer by wafer process at a stroke.
The pitch-conversion should be done in a space in a package(case) but out of the (silicon) bench. The package should contain fiber pitch-expanding region. The fiber pitch-extending region may be a free space for allowing the pitch to extend in a free state. Since the fibers are held by the connector and the bench at both ends, the fibers bend into natural curvatures in the free state. The fibers are later fixed with a resin in the package. The curving portions are sustained by the resin stably without vibration in the package.
Alternatively, V-grooves can be prepared in the pitch-extending region in an inner case for guiding fibers in predetermined curvatures. The V-grooves determine the curvatures of the fibers. Arbitrary curvatures can be given to the fibers. Preparation of the V-grooves on the case is more facile than formation of curved lightwaveguides on a bench. The V-grooves can be formed, when the inner cases are produced by moulding. Fibers should be fitted into the V-grooves on the inner case and fixed with a resin.
Instead of the curvatures made on the inner case, a plurality of protrusions can be prepared on a bottom wall of the inner case for guiding fibers in predetermined curvatures. The protrusions determine the curvatures of the fibers. Like the V-grooves, the set of protrusions gives arbitrary curvatures to the fibers. In this case, the fibers should be fixed stably in the case with a resin.
250 xcexcm is the fiber pitch P1 in MT connectors and ribbonfibers (tapefibers). The V-grooves or the lightwaveguides made on silicon benches have a pitch P2 of 500 xcexcm to 1500 xcexcm, which is determined by the necessary space of mounting laser diode or photodiode chips with adequate margins. An optimum pitch P2 should be chosen by taking account of the sizes of optoelectronic chips and suppression of crosstalk. Leadpins should have been built-in in package by insert-molding for wirebonding leadpins to the optoelectronic chips or metallized wirings. A connector and a silicon bench should be fitted into the inner case. A module is completed by encapsulating the connector, bench and the case into a plastic package by transfermolding with a rigid resin in a metallic mold.
This invention reconciles a wider chip-requiring pitch P2 with a narrower multichannel pitch P1 by curving fibers in a free space between a connector and a bench. Thin optical fibers, which are rich in elasticity by nature, enable modules to form a suitable curving multichannel paths in the free space. On the contrary, formation of curving dielectric lightwaveguides on the bench requires difficult fabrication and long benches, which leads to high cost modules. Instead of lightwaveguides, the present invention employs the optical fibers for changing pitches. The cost for curving lightpaths is zero in the case of fiber paths. Abundant elasticity of the fibers curtails the necessary length of the bench. Use of short benches allows the present invention to alleviate a rise of cost for multichannel modules. The bench can be made of silicon, ceramics or plastics. The cost reduction is conspicuous, in particular, for expensive single crystal silicon bench.
The present invention gives low-cost multichannel modules which harmonize the wide chip arrangement pitch P2 with the standardized channel pith P1 by assigning the optical fibers to making the curving lightpaths. Curving of the fibers in the free space is far easier than producing of curving lightwaveguides on the bench. Although the fibers bend once in the free state, the fibers are fixed with an adhesive in a package. The curved fibers neither flutter nor flap in the module.
An available interface between a multichannel module and an outer multichannel network is a prevalent MT connector which has a pitch P1=250 xcexcm=0.25 mm. The module ensures a sufficiently wide pitch P2=500 xcexcm to 1000 xcexcm for mounting laser or photodiode chips in an inner space of the package.
The present invention proposes low cost multichannel optical communication modules of high reliability and facile fabrication. The package can be formed by transfermolding the bench and the connector with a hard resin, which is suitable for mass scale production.